There have been proposed a large number of refrigerators aimed at saving energy and improving the refrigerator's ability to refrigerate quickly by making the rotational frequency or speed of the compressor variable. For example, as disclosed in Japanese Laid-Open Patent Publication (unexamined) No. 2-140577, there is a trial of producing an effect by making the rotational frequency of a compressor of a refrigerator variable by means of an inverter.
A rotary compressor, as disclosed in the aforementioned prior art document, has generally been used as a compressor whose rotational frequency is made variable by an inverter. The rotary compressor has been so used because its refrigerating ability varies approximately linearly according to the chance of the rotational frequency and because it has had an excellent capability in that its lubricating ability depends less on the rotational frequency.
However, according to the conventional construction, there has been the following problems in using the rotary compressor.
In general, the rotary compressor has a high pressure inside its shell. That is, an inhalation gas having a low pressure is directly inhaled into a cylinder of its compressing section. The gas is then discharged once into the shell after compression. Thereafter, the gas is transferred into a cooling system through a discharge pipe. Thus, since the shell has a high internal pressure, it has been widely known that the gas having a high pressure and a high temperature leaks causing the gas to intrude into a cylinder inside the compression section. This leakage and intrusion is a factor in the reduction of the compression efficiency of the compressor (leakage heat loss).
However, the leakage heat loss has no relation to the rotational frequency, and depends on the magnitude of the high pressure and the magnitude of the low pressure. That is, there has been such a phenomenon that, when the rotational frequency has been lowered to reduce the refrigerating ability of the compressor itself, the rate of the leakage heat loss has increased, consequently reducing the efficiency of the compressor.
When the internal temperature of the refrigerator is stabilized, the need for great refrigerating ability is not present. In such a case, where energy saving is attempted by lowering the rotational frequency by an inverter to reduce the refrigerating ability, there has been a problem in that the energy saving effect cannot be obtained due to the reduction of the efficiency of the compressor.
Furthermore, in the case of a reciprocating compressor, the oil supplying ability depends on the rotational frequency. This dependence has caused a problem in that the reliability is degraded particularly at a low rotational frequency. Also, because the reciprocating compressor requires a large starting torque, smooth starting has not been able to be achieved.
Also, there has been proposed a method of starting a compressor motor control device wherein the position of rotor magnetic poles of a DC brushless motor, whose rotational frequency is controlled by an inverter, is detected by utilizing an induction voltage at the stator winding in a sensor-less system. This method, however, cannot effect the position detection when the motor is stopped, because no induction voltage is generated in such a state. Therefore, it has been a general practice to execute the starting according to a predetermined starting sequence pattern up to a specified rotational frequency at which the position detection is enabled, and thereafter to switch the pattern to the sensor-less system. Such a prior art starting method for the compressor motor control device is disclosed, for example, in Japanese Laid-Open Patent Publication (unexamined) No. 1-54960.
Because a transient DC component in a filter circuit employed in a sensor-less circuit is not sufficiently attenuated at the starting of the DC motor, the above method has been devised to prevent the possible failure of the switching as a consequence of an unstable switching to the sensor-less system. According to this method, the switching to the sensor-less system is effected after the transient DC component is sufficiently attenuated in order to reduce starting failures of the compressor motor control device.
This method, however, uses only one starting sequence pattern, which has caused a problem in that, when a load torque of the DC motor is great at the time of starting, the compressor is occasionally brought into a locked state during the starting sequence pattern operation before the switching to the sensor-less system is effected.
On the other hand, brushless motors have been widely used since they have high efficiencies and permit a rotational frequency control under voltage control. Particularly, since the method of detecting the rotational position from a reverse induction voltage generated at the winding voltage of the motor was proposed lately as a technique for obviating the need of a position detection element for detecting the rotational position of the brushless motor, brushless motors have been extensively used even in very bad operational environments such as with compressors and the like where the temperature is high and refrigerant and oil exist inside.
Generally, in order to eliminate the influence of a voltage waveform due to PWM (Pulse Width Modulation) in detecting the reverse induction voltage, filter circuits are often used. Such use of filter circuits however, has caused a problem in that the position detection becomes unstable in a transient state such as the motor starting stage. A method for eliminating the above disadvantage has been also proposed, for example, in Japanese Patent Laid-Open Publication (unexamined) No. 58-190287. The prior art brushless motor starting method will be described below with reference to FIG. 19.
FIG. 19 is an explanatory view of a prior art brushless motor starting method.
Referring to FIG. 19, when a stopped motor is started, the motor is operated as a synchronous motor because no reverse induction voltage is generated (low-frequency synchronous starting). In this stage, a drive frequency is accelerated so that the rotational frequency gradually increases. With this operation, the rotational frequency also increases.
When the rotational frequency of the motor reaches a specified rotational frequency, it is allowed to execute position detection from the reverse induction voltage, and the motor comes to operate as a brushless motor by switching. Thereafter, acceleration, deceleration and maintaining of the rotational frequency can be achieved by controlling the voltage.
By providing time intervals (t4 and t5) in which no acceleration is effected for a specified time in the switching stage and effecting the switching after waiting for a sufficient attenuation of the transient DC component in the filter circuit, or by starting acceleration after the transient phenomenon in the switching operation is completed, a stability in the switching stage has been assured.
However, the prior art construction has had the following problems.
In the brushless motor in which the position detection is executed based on the reverse induction voltage, the motor starts its operation as a synchronous motor according to the low-frequency synchronous starting in the motor starting stage. In this stage, a voltage and a frequency are applied to the motor so that a specified torque is generated. In this stage, since noise and vibration are caused when the torque is made excessively high and step-out may be incurred when the torque is insufficient, there is a scheme of applying the voltage and frequency in the most appropriate state as far as possible.
Furthermore, in the position detection operation based on the reverse induction voltage, the filter circuit is originally designed so as to become optimum in a region where the motor operates normally, and therefore, the motor tends to step out when a high torque is applied at a low speed.
Accordingly, the prior art method has been effective for a motor that has a small load in the starting stage or at a low rotational frequency (e.g., fan motor).
However, in compressors for use in refrigerators and air conditioners or the like, there may be a case where a high load is applied several seconds after the starting stage. Generally, in a compressor, a difference in pressure of compression gas takes place and the load torque increases immediately after the starting. Particularly, it is well known that a great amount of torque is applied several seconds after the starting stage.
When the conventional method is used in such a case, since the acceleration stop interval is provided when a high load torque is applied, there has been such a problem that a step-out is caused by the high load torque in either the low-frequency synchronous starting operation or the operation based on the reverse induction voltage detection.
Particularly, at the time of turning on the power, capacitors of the filter circuit are totally completely discharged and, therefore, a considerable duration of the acceleration stop interval has been required until the motor is put into its stable state. Accordingly there has been a problem in that the motor tends to step out in the acceleration stop interval.
The present invention has been devised in view of the aforementioned problems inherent in the prior art techniques. It is, accordingly, an object of the present invention to provide a refrigerating apparatus which fails little at the time of starting. When a locked state of the compressor is detected during a starting sequence pattern operation in the refrigerating apparatus of the present invention, the compressor is restarted according to a starting sequence pattern of an output torque that is greater by one step.
Another object of the present invention is to provide a refrigerating apparatus which fails less at the time of starting by detecting a load torque of a DC motor based on the ambient temperature of a refrigerating system, a cooler temperature or an inhalation pressure, and by starting the motor according to a starting sequence pattern corresponding to the load torque from the beginning of operation.
A further object of the present invention is to provide a refrigerator control device capable of preventing the possible reduction in efficiency of the compressor due to the leakage heat loss, assuring a high efficiency even at a low rotational frequency, and remarkably reducing the amount of power consumption.
A still further object of the present invention is to provide a refrigerator control device capable of stably starting the compressor by generating a specified torque and executing a stable operation without incurring step-out just after the starting.
Another object of the present invention is to provide a refrigerator control device having an improved reliability by speedily executing oil supply at the time of starting and assuring a sufficient amount of lubricating oil when oil shortage occurs due to the occurrence of an unforeseen accident such as mixture of gas in the stage of slow rotation.
A further object of the present invention is to provide a brushless motor starter capable of operating the motor without step-out even when a high load torque is required after the starting by sufficiently reducing the transient DC component without providing any acceleration stop interval.
A still further object of the present invention is to provide a brushless motor starter capable of stably starting the motor by speedily completing a position detection process even at the time of closing the power during which the position detection is likely to be unstable, or by compulsorily terminating the process even when the process is not completed by decision.